This invention relates to microelectronic devices and fabrication methods, and more particularly to gallium nitride semiconductor devices and fabrication methods therefor.
Gallium nitride is being widely investigated for microelectronic devices including but not limited to transistors, field emitters and optoelectronic devices. It will be understood that, as used herein, gallium nitride also includes alloys of gallium nitride such as aluminum gallium nitride, indium gallium nitride and aluminum indium gallium nitride.
A major problem in fabricating gallium nitride-based microelectronic devices is the fabrication of gallium nitride semiconductor layers having low defect densities. It is known that one contributor to defect density is the substrate on which the gallium nitride layer is grown. Accordingly, although gallium nitride layers have been grown on sapphire substrates, it is known to reduce defect density by growing gallium nitride layers on aluminum nitride buffer layers which are themselves formed on silicon carbide substrates. Notwithstanding these advances, continued reduction in defect density is desirable.
It also is known to produce low defect density gallium nitride layers by forming a mask on a layer of gallium nitride, the mask including at least one opening therein that exposes the underlying layer of gallium nitride, and laterally growing the underlying layer of gallium nitride through the at least one opening and onto the mask. This technique often is referred to as xe2x80x9cEpitaxial Lateral Overgrowthxe2x80x9d (ELO). The layer of gallium nitride may be laterally grown until the gallium nitride coalesces on the mask to form a single layer on the mask. In order to form a continuous layer of gallium nitride with relatively low defect density, a second mask may be formed on the laterally overgrown gallium nitride layer, that includes at least one opening that is offset from the opening in the underlying mask. ELO then again is performed through the openings in the second mask to thereby overgrow a second low defect density continuous gallium nitride layer. Microelectronic devices then may be formed in this second overgrown layer. ELO of gallium nitride is described, for example, in the publications entitled Lateral Epitaxy of Low Defect Density GaN Layers Via Organometallic Vapor Phase Epitaxy to Nam et al., Appl. Phys. Lett. Vol. 71, No. 18, Nov. 3, 1997, pp. 2638-2640; and Dislocation Density Reduction Via Lateral Epitaxy in Selectively Grown GaN Structures to Zheleva et al, Appl. Phys. Lett., Vol. 71, No. 17, Oct. 27, 1997, pp. 2472-2474, the disclosures of which are hereby incorporated herein by reference.
It also is known to produce a layer of gallium nitride with low defect density by forming at least one trench or post in an underlying layer of gallium nitride to define at least one sidewall therein. A layer of gallium nitride is then laterally grown from the at least one sidewall. Lateral growth preferably takes place until the laterally grown layers coalesce within the trenches. Lateral growth also preferably continues until the gallium nitride layer that is grown from the sidewalls laterally overgrows onto the tops of the posts. In order to facilitate lateral growth and produce nucleation of gallium nitride and growth in the vertical direction, the top of the posts and/or the trench floors may be masked. Lateral growth from the sidewalls of trenches and/or posts also is referred to as xe2x80x9cpendeoepitaxyxe2x80x9d and is described, for example, in publications entitled Pendeo-Epitaxy: A New Approach for Lateral Growth of Gallium Nitride Films by Zheleva et al., Journal of Electronic Materials, Vol. 28, No. 4, February 1999, pp. L5-L8; and Pendeoepitaxy of Gallium Nitride Thin Films by Linthicum et al., Applied Physics Letters, Vol. 75, No. 2, July 1999, pp. 196-198, the disclosures of which are hereby incorporated herein by reference.
Pendeoepitaxy can provide relatively large, low defect gallium nitride layers for microelectronic applications. Unfortunately, it has been found that pendeoepitaxially fabricated gallium nitride layers can have low defect densities during fabrication, but can exhibit cracks and other defects after fabrication. In particular, since the pendeoepitaxial layer generally is formed on a non-gallium nitride substrate, stress may occur in the pendeoepitaxial layer due to thermal expansion coefficient mismatch between the substrate and the pendeoepitaxial gallium nitride layer as the temperature is reduced from the elevated growth temperature to room temperature. This stress due to thermal expansion coefficient mismatch may create cracks and/or other defects in the gallium nitride semiconductor layer, which can greatly reduce the suitability thereof for microelectronic device applications.
Moreover, since the pendeoepitaxial gallium nitride layer is formed on a substrate, it may be difficult to provide a freestanding gallium nitride semiconductor layer that can be used as a large area seed for further gallium nitride bulk growth. The substrate can be removed to provide a freestanding gallium nitride semiconductor layer, but substrate removal may be difficult using conventional techniques, without damaging the gallium nitride semiconductor layer.
Accordingly, notwithstanding the recent advances of pendeoepitaxy, there continues to be a need for methods of fabricating gallium nitride semiconductor layers having low defect densities at room temperature and for fabricating freestanding gallium nitride semiconductor layers.
The present invention pendeoepitaxially grows a gallium nitride layer on weak posts on a substrate that are configured to crack due to a thermal expansion coefficient mismatch between the substrate and the gallium nitride layer on the weak posts. Thus, upon cooling, at least some of the weak posts crack, to thereby relieve stress in the gallium nitride semiconductor layer. Accordingly, low defect density gallium nitride semiconductor layers may be produced. Moreover, the weak posts can allow relatively easy separation of the substrate from the gallium nitride semiconductor layer to provide a freestanding gallium nitride layer.
More specifically, gallium nitride semiconductor layers may be fabricated by forming a plurality of weak posts on a substrate. The weak posts define a plurality of sidewalls and are configured to crack due to a thermal expansion coefficient mismatch between the substrate and the later formed gallium nitride semiconductor layer on the weak posts. A gallium nitride layer is grown from the sidewalls of the weak posts at elevated temperature, until the gallium nitride layer coalesces to produce a gallium nitride semiconductor layer. The gallium nitride layer preferably is grown using pendeoepitaxy so that the gallium nitride layer is cantilevered from the substrate. At least some of the weak posts then are cracked due to the thermal expansion coefficient mismatch between the substrate and the gallium nitride semiconductor layer upon reducing the elevated temperature. Stress in the gallium nitride semiconductor layer thereby can be relieved.
The weak posts may be formed by forming an array of posts in spaced apart staggered relation on the substrate. By staggering the posts, later fracturing may be promoted compared to long unstaggered posts. Alternatively, the posts may have a height to width ratio in excess of 0.5, so that the relatively narrow posts promote cracking upon reduction of the temperature. In another alternative, the posts preferably are less than one micron wide, more preferably less than one half micron wide, regardless of height, to promote cracking. In yet another alternative, a post weakening region is formed in the posts, adjacent the substrate. In particular, a buried region may be formed in the substrate and the substrate then may be selectively etched to define the plurality of weak posts including the post weakening regions that comprise the buried region. The buried region may comprise implanted ions, preferably hydrogen ions, that can agglomerate to form hydrogen bubbles within the posts that can fracture the posts upon cooling. It will be understood that each of the above-described techniques of staggered posts, narrow posts and post weakening regions may be used separately or in combination to produce weak posts on a substrate according to the present invention.
At least some of the weak posts crack due to the thermal expansion coefficient mismatch between the substrate and the gallium nitride semiconductor layer upon reducing the elevated temperature, to thereby relieve stress in the gallium nitride semiconductor layer. Thus, the pendeoepitaxial substrates act as an engineered weak platform. Instead of cracks occurring throughout the gallium nitride semiconductor layer, the cracks preferably occur at the posts and may actually shear some of the posts, leaving the gallium nitride semiconductor layer intact. Moreover, all of the weak posts may crack and/or shear, to thereby separate the gallium nitride semiconductor layer from the substrate and produce a freestanding gallium nitride semiconductor layer. Alternatively, the weak posts may facilitate the separation of the gallium nitride semiconductor layer from the substrate at the weakened posts, to produce a freestanding gallium nitride semiconductor layer. The freestanding gallium nitride semiconductor layer then may act as a large area seed for subsequent epitaxial growth of a gallium nitride layer on the freestanding gallium nitride semiconductor layer.
Gallium nitride semiconductor structures according to the present invention include a substrate, a plurality of posts on the substrate that include a plurality of sidewalls, and a gallium nitride semiconductor layer extending between the sidewalls of adjacent posts. At least one of the posts is cracked between the substrate and the gallium nitride layer. The plurality of posts may be in spaced apart staggered relation on the substrate. The plurality of posts may have a height to width ratio in excess of 0.5. The plurality of posts may be less than one micron and more preferably less than one half micron wide. The plurality of posts may include a post weakening region therein adjacent the substrate that contains bubbles, preferably hydrogen bubbles, therein.
Moreover, the present invention may provide a freestanding monocrystalline gallium nitride substrate having first and second opposing faces each having an area greater than 0.25 cm2 and having a defect density of less than 105 cmxe2x88x922. The structures may include at least one post extending from one of the faces, wherein the post is jagged. An array of spaced apart staggered posts, bubble posts and/or posts that are less than one half micron wide, may be provided. An epitaxial gallium nitride layer also may be provided on one of the first and second opposing faces of the freestanding gallium nitride layer. Accordingly, gallium nitride semiconductor layers that can exhibit reduced susceptibility to cracking after fabrication and relatively large area freestanding gallium nitride layers may be provided.